1. Field of the Invention
The present invention relates to a technique of annealing, for instance, a semiconductor material uniformly and efficiently over a large area. The invention also relates to a technique of preventing reduction of processing efficiency in illuminating a particular region while gradually changing the illumination energy density.
2. Description of the Related Art
In recent years, extensive studies have been made of the temperature reduction of semiconductor device manufacturing processes. This is largely due to the need of forming semiconductor devices on an insulative substrate, such as a glass substrate, which is inexpensive and superior in workability. Other needs such as needs of forming finer devices and multilayered devices have also prompted the studies on the process temperature reduction.
In particular, a technique of forming semiconductor devices on a glass substrate is necessary to produce a panel that constitutes an active matrix liquid crystal display device. This is a configuration in which thin-film transistors are formed on a glass substrate so as to assume a matrix of more than several hundred by several hundred. When a glass is exposed to an atmosphere of more than about 600xc2x0 C., deformation such as contraction and strain becomes remarkable. Therefore, the heating temperature in a thin-film transistor manufacturing process should be as low as possible.
To obtain thin-film transistors having superior electrical characteristics, a crystalline thin-film semiconductor needs to be used.
Among methods of producing a crystalline silicon film is a technique of crystallizing, by a heat treatment, an amorphous silicon film that has been deposited by plasma CVD or low-pressure thermal CVD of about 500xc2x0 C. This heat treatment is such that a sample is left in an atmosphere of 600xc2x0 C. or more for more than several hours. In this heat treatment, where the temperature is, for instance, 600xc2x0 C., long process time of more than 10 hours is needed. In general, if a glass substrate is heated at 600xc2x0 C. for more than 10 hours, deformation (strain and contraction) of the substrate becomes remarkable. Since a thin-film semiconductor for constituting thin-film transistors is several hundred angstrom in thickness and several micrometers to several tens of micrometers in size, the substrate deformation will cause an operation failure, a variation in electrical characteristics, or the like. In particular, in the case of a large-sized substrate (diagonal size: 20 inches or more), the substrate deformation is a serious problem.
If the heat treatment temperature is higher than 1,000xc2x0 C., crystallization can be attained in a process time of several hours. However, ordinary glass substrates cannot withstand a high temperature of about 1,000xc2x0 C. even if a heat treatment lasts for a short time.
Quartz substrates can withstand a heat treatment of more than 1,000xc2x0 C., and allow production of a silicon film having superior crystallinity. However, large-area quartz substrates are particularly expensive. Therefore, from the economical point of view, they cannot be easily applied to liquid crystal display devices, which will be required to be increased in size in the future.
In the above circumstances, the temperature of processes for manufacturing thin-film transistors is now required to be lowered. Among techniques for attaining this purpose is an annealing technique that uses laser light illumination, a technique which now attracts much attention with a possibility of providing an ultimate low-temperature process. Since laser light can impart high energy thermal annealing to only a necessary portion, it is not necessary to expose the entire substrate to a high-temperature atmosphere. Therefore, the annealing technique by laser light illumination enables use of glass substrates.
However, the annealing technique by laser light illumination has a problem of unstable laser light illumination energy. Although this problem can be solved by using a laser apparatus capable of emitting laser light of higher energy than necessary and attenuating the output laser light, there remains another problem of cost increase due to increased size of the laser apparatus.
Even with such a problem, the annealing technique by laser light illumination is still very advantageous in that it enables use of glass substrates.
In general, there are two laser light illumination methods described below.
In a first method, a CW laser such as an argon ion laser is used and a spot-like beam is applied to a semiconductor material. A semiconductor material is crystallized such that it is melted and then solidified gradually due to a sloped energy profile of a beam and its movement.
In a second method, a pulsed oscillation laser such as an excimer laser is used. A semiconductor material is crystallized such that it is instantaneously melted by application of a high-energy laser pulse and then solidified.
The first method of using a CW laser has a problem of long processing time, because the maximum energy of the CW laser is insufficient and therefore the beam spot size is at most several millimeters by several millimeters. In contrast, the second method using a pulsed oscillation laser can provide high mass-productivity, because the maximum energy of the laser is very high and therefore the beam spot size can be made several square centimeters or larger.
However, in the second method, to process a single, large-area substrate with an ordinary square or rectangular beam, the beam needs to be moved in the four orthogonal directions, an inconvenience still remaining to be solved from the viewpoint of mass-productivity.
This aspect can be greatly improved by deforming a laser beam into a linear shape that is longer than the width of a subject substrate, and scanning the substrate with such a deformed beam.
The remaining problem is insufficient uniformity of laser light illumination effects. The following measures are taken to improve the uniformity. A first measure is to make the beam profile as close to a rectangular one as possible by causing a laser beam to pass through a slit, to thereby reduce an intensity variation within a linear beam. A second measure to further improve the uniformity is to perform preliminary illumination with pulse laser light that is weaker than that of subsequently performed main illumination. This measure is so effective that the characteristics of resulting semiconductor devices can be improved very much.
The reason why the above two-step illumination is effective is that a semiconductor material film including many amorphous portions has a laser energy absorption ratio that is much different than a polycrystalline film. For example, a common amorphous silicon film (a-Si film) contains hydrogen at 20 to 30 atomic percent. If laser light having high energy is abruptly applied to an amorphous silicon film, hydrogen is ejected therefrom, so that the surface of the film is roughened, i.e., formed with asperities of several tens of angstrom to several hundred angstrom. Since a thin-film semiconductor for a thin-film transistor is several hundred angstrom in thickness, its surface having asperities of several tens of angstrom to several hundred angstrom will be a major cause of variations in electrical characteristics etc.
Where the two-step illumination is performed, a process proceeds such that a certain part of hydrogen is removed from an amorphous silicon film by the weak preliminary illumination and crystallization is effected by the main illumination. Since the illumination energy is not high in the preliminary illumination, there does not occur severe surface roughening of the film due to sudden hydrogen ejection.
The uniformity of the laser light illumination effects can be improved considerably. However, if the above two-step illumination is employed, the laser processing time is doubled, thus reducing throughput. Further, since a pulsed laser is used, some variation occurs in the laser annealing effects depending on the registration accuracy of the main illumination and preliminary illumination, which variation may greatly influence the characteristics of thin-film transistors having a size of several tens of micrometers by several tens of micrometers.
In general, among various processing techniques. (for example, causing a quality change in various materials and processing by application of laser energy) by laser light illumination is a technique in which a certain region is illuminated plural times with laser beams of varied energies. The above-described annealing technique for a silicon film is an example of such a technique.
Conventionally, in such a technique, a laser beam is applied plural times, which however elongates the processing time by a factor of the number of illumination times, and causes a large decrease in the operation efficiency. Further, illuminating a particular region plural times with laser beams likely causes a problem of a deviation of illumination areas, and is not practical because solving this problem may be technically difficult or may require a costly technique.
An object of the present invention is to solve the problem of nonuniformity of the effects of annealing by laser light illumination. Another object of the invention is to improve the economy of laser light illumination.
The invention solves the above problems by devising a new energy profile of a linear laser beam, which profile varies continuously or step-like manner. Specifically, a normal-distribution type profile or a trapezoidal profile is employed.
To solve the above problems, one aspect of the invention is characterized in that an illumination object is illuminated with pulse laser beams that have been shaped into linear beams while being scanned with the laser beams relatively in one direction.
For example, as shown in FIG. 3, a semiconductor material is illuminated, while being scanned, with a laser beam having a normal-distribution type energy profile in its width direction (i.e., scanning direction). With this illumination method, a foot-to-middle portion of the normal-distribution type profile corresponds to the preliminary illumination having low laser beam energy, while a middle-to-top portion of the profile corresponds to the main illumination having high energy. Therefore, a single laser beam illuminating operation can provide effects similar to those obtained by the two-step or multi-step laser beam illumination. Alternatively, as shown in FIG. 5, a semiconductor material is illuminated with a laser beam having a trapezoidal energy profile in its width direction (scanning direction). In this case, a slope portion of the trapezoidal profile has a function of imparting energy corresponding to that of the preliminary illumination, while a top base portion of the profile has a function of imparting energy corresponds to that of the main illumination.
Another aspect of the invention is characterized in that an illumination object is illuminated with pulse laser beams that have been shaped into linear beams while the laser beams are moved in one direction, in which the laser beams are applied in an overlapped manner so that an arbitrarily selected point on the illumination object is illuminated plural times.
In this method, a particular region is illuminated with laser beams plural times by applying linear laser beams in an overlapped manner.
In particular, where laser beams having a normal-distribution type energy profile (see FIG. 3) or a trapezoidal energy profile (see FIG. 5) in the scanning direction are applied in an overlapped manner while being moved little by little, in a particular linear region the applied energy density first increases continuously or in a step-like manner and then decreases continuously or in a step-like manner. Therefore, this method can provide effects similar to those obtained by the two-step or multi-step laser light illumination.
To provide effects equivalent to those obtained by the multi-step illumination, the number of overlappings of laser beam pulses may be set at 3 to 100, preferably 10 to 30.
However, to obtain necessary annealing effects, it is preferred that the laser beam illumination is so performed as to satisfy certain conditions, which are:
(1) The illumination object is a silicon film of 150 to 1,000 xc3x85 in thickness.
(2) The laser beams are pulse beams having a pulse rate of N per second, assumes a linear shape having a width L, and has a beam profile in which the energy density varies continuously or in a step-like manner in the width direction.
(3) The laser beams are applied to an illumination surface while being moved at a speed V in the width direction.
(4) The average single-pulse energy density is set at 100 to 500 mJ/cm2.
(5) The laser beams are applied so as to satisfy a relationship 10xe2x89xa6LN/Vxe2x89xa630.
A laser beam illumination method of the invention which satisfies the above conditions is described as comprising the steps of:
emitting pulse laser beams at a rate of N times per second;
shaping the pulse laser beams into linear beams having a width L, an energy profile that varies continuously or in a step-like manner in a width direction thereof, and an average single-pulse energy density of 100 to 500 mJ/cm2; and
applying the laser beams to a silicon film having a thickness of 150 to 1,000 xc3x85 while scanning it with the laser beams in the width direction at a speed V so as to satisfy a relationship 10xe2x89xa6LN/Vxe2x89xa630.
Among the above conditions, the condition that the illumination object is a silicon film of 150 to 1,000 xc3x85 in thickness is established for the following reasons. Experiments have shown that in annealing of a silicon film, if the thickness of the silicon film is less than 150 xc3x85, the uniformity of film formation, the uniformity of annealing effects, and the reproducibility are insufficient. On the other hand, a silicon film having a thickness of more than 1,000 xc3x85 is not practical because it requires a large-output laser. In addition, a crystalline silicon film having such a thickness is not used for a thin-film transistor.
Examples of the laser beam having a beam profile that varies continuously or in a step-like manner in the width direction are a laser beam having a normal-distribution type energy profile in the scanning direction (see FIG. 3) and a laser beam having a trapezoidal energy profile in the scanning direction (see FIG. 5).
The reason for employing the energy density of 100 to 500 mJ/cm2 is that experiments have revealed that laser annealing of a silicon film having a thickness of not more than 1,000 xc3x85 can be performed effectively by using a laser beam of the above energy density. The energy density as used above is defined as the value of a top portion of a profile that varies continuously or in a step-like manner. For example, in the case of a normal-distribution type profile, the energy density is defined as the maximum value. In the case of a trapezoidal profile, the energy density is defined as the value of a top base portion.
In the above method, the parameter LN/V represents the number of laser beam pulses applied to a particular linear region when it is scanned with linear pulse laser beams once. To attain the effects as obtained by the multi-step illumination, it is preferred that the number of laser beam pulses be set at 10 to 30.
According to a further aspect of the invention, there is provided a laser beam illumination method comprising the steps of:
emitting pulse laser beams at a rate of N times per second;
shaping the pulse laser beams so that they have an energy profile in which an energy density varies continuously or in a step-like manner over a length L in a predetermined direction; and
applying the laser beams to a predetermined region while scanning it with the laser beams in the predetermined direction at a speed V, wherein the number n of laser beam pulses applied to the predetermined region in one scan satisfies a relationship n=LN/V.
By employing the above method, a particular region can be illuminated n times with laser beams whose energy density gradually varies.
In the invention, the laser beam illumination energy profile is not limited to normal-distribution type and trapezoidal profiles. For example, there may be employed a beam shape in which the energy density varies in a step-like manner, or a triangular energy profile may be used.
For example, when a linear laser beam having a normal-distribution type energy profile as shown in FIG. 3 is applied while being moved for scanning in its width direction so that certain conditions are satisfied, first a weak foot portion of the energy profile is applied and the illumination energy gradually increases. After a portion having a certain energy value is applied, the illumination energy gradually decreases and the illumination is finished.
For example, when linear pulse laser beams having a normal-distribution type illumination energy profile in the width direction are used and a condition LN/V=15 is satisfied where L is a beam width, N is the number of emissions per second, and V is a scanning speed, a linear region is illuminated with 15 laser beam pulses in one laser beam scan. The 15 laser beam pulses, which are sequentially applied, have energy density values of 15 sections of the normal-distribution type profile, respectively. For example, a particular linear region (the width of this region is very narrow) is sequentially illuminated with laser beam pulses having energy density values E1 to E15 shown in FIG. 4. As the laser beam pulses of E1 to E8 are sequentially applied, the illumination energy density gradually increases. On the other hand, as the laser beam pulses of E8 to E15 are sequentially applied, the illumination energy density gradually decreases.
A process of this type in which first the illumination energy is gradually increased and then gradually decreased can attain desired annealing effects while suppressing surface roughening of a silicon film. Further, since desired effects can be obtained by laser beam illumination of one scan rather than plural times of laser beam illuminating operations, high operation efficiency can be attained.
In particular, effects similar to those as obtained by the multi-step illumination can be attained by using laser beams whose energy density is varied continuously. This function similarly applies to effects other than the annealing effects on a silicon film.